1. Field of the Invention
The present invention relates to a forward/backward switching control apparatus for a hydraulic drive vehicle and a control method therefor and, more particularly, to a forward/backward switching control apparatus for a hydraulic drive vehicle, which is suited for a hydraulic drive vehicle to work while running, and a control method therefor.
2. Description of the Related Art
In the hydraulic drive vehicle, there has been generally used a construction, in which a variable displacement type hydraulic pump and a variable displacement type hydraulic motor or a fixed displacement type hydraulic motor are configured in a closed circuit.
Especially, the tire type hydraulic drive vehicle uses the variable displacement type hydraulic pump and the variable displacement type hydraulic motor to improve its controllability and runnability and so on. In order to prevent shocks and cavitations at the stopping time or at the turning time, it is the current practice that the tire type hydraulic drive vehicle is equipped with a safety valve and that the charge pressure is fed to the closed circuit.
In recent years, there has been used in a working vehicle or the like a forward/backward switching control apparatus which can be switched from forward to backward or from backward to forward, while the vehicle is running, directly by a control lever. This forward/backward switching control apparatus was proposed in JP-A-11-182674 by the Inventors.
When the hydraulic drive vehicle runs forward, for example, according to that Japanese Patent Application, the discharge oil of a variable displacement type hydraulic pump is fed through the forward position of a direction control valve from a first main circuit to one port of a variable displacement type hydraulic motor, and the discharge oil from the other port of the hydraulic motor is returned from a second main circuit through the forward position of the direction control valve to a tank, so that the hydraulic motor is driven forward.
The direction control valve is a three-position switching valve having a neutral position, and there is interposed between the direction control valve and the tank a back pressure valve for throttling the return oil from the direction control valve to apply a back pressure to the return oil. The direction control valve is switched in response to a pilot pressure oil coming from a direction changing electromagnetic valve to be activated by a command of a controller, to feed a pressure oil of the variable displacement hydraulic pump to the variable displacement type hydraulic motor thereby to run the hydraulic drive vehicle.
The forward/backward switching control apparatus comprises: a vehicle speed sensor for detecting a vehicle speed; a timer for inputting a vehicle speed signal to delay the duration for a controller to input a switching command and to output a switching control signal, by a preset time in accordance with the vehicle speed; a back pressure correction control valve for inputting a motor back pressure correction command from the controller to lower a control oil pressure for controlling the back pressure valve, thereby to correct the back pressure to rise; and an electromagnetic reducing valve for inputting a motor displacement correction command from the controller, to raise a motor displacement control oil pressure for controlling a motor discharge displacement control valve thereby to make such a correction that the discharge displacement of the motor, as might otherwise become the maximum, may be lowered to an arbitrary discharge displacement.
In the aforementioned construction, the controller holds the neutral position for a preset time and then outputs the switching control signal for switching the neutral position to forward or backward, to a direction switching electromagnetic valve.
In this forward/backward switching control apparatus, a pump displacement control valve receives a pump displacement control pressure from the electromagnetic reducing valve to increase the pump discharge displacement. At a deceleration, therefore, the hydraulic motor can be supplied with the increased discharge oil of the hydraulic pump to prevent the cavitation reliably.
When the speed is decelerated to a preset value or lower, the rotation speed of the hydraulic motor is lowered to reduce the supply needed. Therefore, the back pressure valve is returned to lower the back pressure of the return oil to be returned to the tank, thereby to reduce the back pressure loss. When the direction is smoothly switched from neutral to forward or from neutral to backward by increasing the discharge displacement of the hydraulic motor for the deceleration, moreover, the switching shock is reduced.
As a result, the forward/backward switching control apparatus is enabled to perform the forward/backward switching during the running smoothly without any speed changing shock merely by adding the aforementioned construction simply to the construction of the prior art. It is also described that the back pressure is minimized to improve the running efficiency in the running state other than deceleration and that the cavitation of the hydraulic motor can be reliably prevented in the deceleration.